fix: Treat empty TMPDIR as unset

Fixes an instance of

    nix: src/libutil/util.cc:139: nix::Path nix::canonPath(PathView, bool): Assertion `path != ""' failed.

... which I've been getting in one of my shells for some reason.
I have yet to find out why TMPDIR was empty, but it's no reason for
Nix to break.

src/libstore/globals.cc

@@ -415,7 +415,7 @@ void initLibStore() {
        sshd). This breaks build users because they don't have access
        to the TMPDIR, in particular in ‘nix-store --serve’. */
 #if __APPLE__
-    if (hasPrefix(getEnv("TMPDIR").value_or("/tmp"), "/var/folders/"))
+    if (defaultTempDir(), "/var/folders/"))
         unsetenv("TMPDIR");
 #endif
 

src/libutil/file-system.cc

@@ -494,10 +494,16 @@ void AutoDelete::reset(const Path & p, bool recursive) {
 
 //////////////////////////////////////////////////////////////////////
 
+std::string defaultTempDir() {
+    constexpr auto def = "/tmp";
+    auto dir = getEnv("TMPDIR").value_or(def);
+    return dir.empty() ? def : dir;
+}
+
 static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
     std::atomic<unsigned int> & counter)
 {
-    tmpRoot = canonPath(tmpRoot.empty() ? getEnv("TMPDIR").value_or("/tmp") : tmpRoot, true);
+    tmpRoot = canonPath(tmpRoot.empty() ? defaultTempDir() : tmpRoot, true);
     if (includePid)
         return fmt("%1%/%2%-%3%-%4%", tmpRoot, prefix, getpid(), counter++);
     else
@@ -537,7 +543,7 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
 
 std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
 {
-    Path tmpl(getEnv("TMPDIR").value_or("/tmp") + "/" + prefix + ".XXXXXX");
+    Path tmpl(defaultTempDir() + "/" + prefix + ".XXXXXX");
     // Strictly speaking, this is UB, but who cares...
     // FIXME: use O_TMPFILE.
     AutoCloseFD fd(mkstemp((char *) tmpl.c_str()));

src/libutil/file-system.hh

@@ -234,6 +234,10 @@ Path createTempDir(const Path & tmpRoot = "", const Path & prefix = "nix",
  */
 std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix = "nix");
 
+/**
+ * Return `TMPDIR`, or the default temporary directory if unset or empty.
+ */
+Path defaultTempDir();
 
 /**
  * Used in various places.

src/nix-build/nix-build.cc

@@ -476,7 +476,7 @@ static void main_nix_build(int argc, char * * argv)
         auto env = getEnv();
 
         auto tmp = getEnv("TMPDIR");
-        if (!tmp) tmp = getEnv("XDG_RUNTIME_DIR").value_or("/tmp");
+        if (!tmp || tmp->empty()) tmp = getEnv("XDG_RUNTIME_DIR").value_or("/tmp");
 
         if (pure) {
             decltype(env) newEnv;